1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging compositions. More particularly, it concerns oxygen scavenging compositions comprising polymers derived from benzenedimethanol monomers.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking.
Approaches for minimizing the oxygen exposure of packaged products can generally be grouped into two categories. One set of approaches involves scavenging oxygen present in the package as a result of the packaging process. The other set of approaches involves minimizing the entry of oxygen into the package during or after the packaging process.
Minimizing the entry of oxygen into the package after packaging can be pursued by forming one or more layers of the package from a polymer known to possess oxygen barrier properties. Ethylene/vinyl alcohol copolymer (EVOH) has very good oxygen barrier properties, but its oxygen barrier properties are sensitive to moisture and it is relatively expensive. Polyethylene terephthalate (PET) does not have the latter disadvantages, but its oxygen barrier properties are not as good as those of EVOH. Therefore, there is interest in preparing modified PET or blends of PET with other polymers that may have better oxygen barrier properties than PET alone without suffering from other shortcomings.
One approach that has been attempted is the blending of PET with an oxygen scavenging polymer. The oxygen scavenging polymer would scavenge oxygen that the PET would otherwise permit to pass from the environment to the package contents. An example of this approach is reported by Cochran et al., U.S. Pat. No. 5,021,515 (“Cochran”), which reports a package comprising a layer comprising a blend of 96 wt % PET, 4 wt % poly(m-xylyleneadipamide) (MXD6), and 200 ppm cobalt. The PET provides oxygen barrier properties; the MXD6 provides oxygen scavenging properties; and the cobalt catalyzes oxygen scavenging by the MXD6.
However, the package of Cochran has a number of shortcomings. First, PET and MXD6 are somewhat incompatible, and as a result, the clarity of a transparent bottle comprising this layer will deteriorate over time. Second, the compounding process requires an undesirably high processing temperature because of the incompatibility issue described above as well as the relatively high melting point of MXD6 relative to PET. Third, an extra thermal solidating process is often required to provide adequate oxygen scavenging performance of the PET/MXD6 blend.
Therefore, it is desirable to have a composition comprising PET and oxygen scavenging moieties with superior compatibility and ease of processing. Such a composition would be expected to impart superior physical properties to a package, especially a bottle, made therefrom.